JavaScript Async Interview Questions concepts including asynchronous programming, event loop, memory management, and security best practices. Boost your frontend interview prep with real-world insights.
⏳ JavaScript Async Interview Questions
66. What is asynchronous programming in JavaScript?
Answer:
Asynchronous programming allows tasks like API calls or timers to run without blocking the main thread.
67. What are callbacks in JavaScript?
Answer:
A callback is a function passed as an argument to another function, to be executed later.
function greet(name, callback) {
console.log("Hello " + name);
callback();
}
greet("Alice", () => console.log("Callback executed!"));
68. What is a Promise in JavaScript?
Answer:
A Promise represents a value that may be available now, later, or never.
let promise = new Promise((resolve, reject) => {
setTimeout(() => resolve("Done"), 1000);
});
promise.then(console.log);
69. Promise states
Pending: Initial state.Fulfilled: Completed successfully.Rejected: Failed.
70. What is async/await?
Answer:async/await is syntactic sugar over Promises to write asynchronous code in a synchronous style.
async function getData() {
const data = await fetch("https://api.com/data");
return await data.json();
}
71. Difference between async/await and Promises?
| Feature | Promises | async/await |
|---|---|---|
| Syntax | Chain .then() | Synchronous-like |
| Readability | Less clean for complex chains | More readable |
| Error handling | .catch() | try...catch |
🔁 Event Loop and Concurrency
72. What is the JavaScript Event Loop?
Answer:
The event loop is a mechanism that handles asynchronous callbacks using a queue (macro/microtasks) and the call stack.
73. How does the call stack work?
Answer:
The call stack keeps track of function execution. When a function is called, it’s pushed to the stack. When it returns, it’s popped off.
74. What are microtasks and macrotasks?
- Microtasks: Promise callbacks,
queueMicrotask - Macrotasks:
setTimeout,setInterval, I/O
Execution order:
- Call stack runs to completion.
- Microtasks queue executes.
- Macrotasks queue executes.
🧼 JavaScript Memory Management and JavaScript Async Interview Questions
75. How is memory managed in JavaScript?
Answer:
Memory is allocated when variables and objects are created and freed when they are no longer used (garbage collected).
76. What is garbage collection in JavaScript?
Answer:
Garbage collection is automatic memory management that reclaims memory occupied by unused objects.
77. Common causes of memory leaks?
- Global variables
- Forgotten timers or callbacks
- Detached DOM nodes
- Closures holding unnecessary references
🔐 JavaScript Security Best Practices and JavaScript Async Interview Questions
78. What is XSS and how to prevent it in JavaScript?
Answer:
XSS (Cross-site scripting) injects malicious scripts into a site. Prevent with:
- Output encoding
- Use innerText instead of innerHTML
- CSP (Content Security Policy)
79. What is CSRF and how is it mitigated?
Answer:
CSRF (Cross-site Request Forgery) tricks users into performing actions without their knowledge. Prevent with:
- CSRF tokens
- SameSite cookies
- Avoiding GET for state-changing actions
80. How to securely use eval() or avoid it?
Answer:
Avoid eval() entirely — it executes code as a string and opens XSS vulnerabilities. Use JSON and safer parsing alternatives.
📋 JavaScript Interview Preparation Checklist
Here’s a handy checklist for your next frontend interview:
✅ Understand the basics: variables, types, operators
✅ Master functions, scope, closures, and hoisting
✅ Deep dive into objects, arrays, and classes
✅ Know the difference between let, var, and const
✅ Handle async tasks with promises, async/await
✅ Practice real coding tasks (palindrome, reverse, flatten, debounce)
✅ Learn the event loop and memory management
✅ Explore browser APIs: DOM, events, storage
✅ Stay sharp on ES6+ features
✅ Follow security best practices
JavaScript Async Interview Questions
🎯 Conclusion
You now have a complete, deep-dive reference for JavaScript interview questions and answers — covering everything from beginner basics to advanced concepts like the event loop and memory leaks.
This five-part guide was designed to help frontend developers, JavaScript engineers, and full-stack candidates prepare for real-world interviews and technical screens. JavaScript Async Interview Questions
✅ Recap of All Parts:
- [Part 1]: JavaScript Basics, Data Types, Variables
- [Part 2]: Functions, Scope, Closures, Hoisting, ES6
- [Part 3]: OOP, Functional Programming, DOM, Events
- [Part 4]: Built-in Methods, Map/Set, JSON, Modules
- [Part 5]: Async JS, Event Loop, Memory, Security, Tips
